1. Field of the Invention
This invention relates to a method for producing a field emission cathode device, more particularly to a method for producing a carbon nanotube array element of a field emission cathode device and the carbon nanotube array element produced therefrom.
2. Description of the Related Art
Carbon nanotube (CNT) has excellent electrical, mechanical, chemical, physical, and thermal properties because of inert and perfect graphene structure and high aspect ratios. In particular, CNT is applied to produce a field emission display device because of low turn-on and threshold voltage. CNT grown by chemical vapor deposition (CVD) techniques is used as an electron emission source. After externally applying a voltage, electrons are emitted from CNT tips and hit an anode (e.g., indium tin oxide substrate having phosphor coating) to generate light. Under an applied voltage of 3 V/μm, the current density is 1 mA/cm2, and the lifetime is up to ten thousands hours. Moreover, compared to other displays, the field emission display (FED) has advantages of high brightness, wide view angle, small thickness, and low energy consumption.
Presently, a vertically aligned CNT array element of a field emission cathode is produced using a template forming method or a screen-printing method.
The template forming method includes providing a substrate having a porous surface, depositing a catalyst layer on the porous surface of the substrate, and placing the substrate with the catalyst layer in a chemical vapor deposition (CVD) furnace with proper controlling of temperature, pressure, and concentration of hydrocarbon gas. Parallel carbon nanotube bundles can be found on the substrate. Examples of the template forming method can be found in U.S. Pat. Nos. 6,512,235 B1 and 6,339,281 B2. Although the carbon nanotubes produced by this method have good alignment, adhesion between the CNTs and the substrate is poor and might result in separation during the field emission operation at high voltage (600 V or more).
To overcome the adhesion problem encountered in the template forming method, a screen-printing method is proposed. A CNT paste consisting of organic bonding agent, resin (e.g., epoxy), carbon nanotubes (CNTs), and silver powder is coated on a substrate using a screen-printing method so as to produce a carbon nanotube field emission cathode having a plurality of electron emitters. The relevant information can be found in U.S. Pat. Nos. 6,436,221 B1, 6,342,276 B1, and 6,146,230. Although the adhesion problem is solved, the precision of the field emission cathode produced by the screen-printing method is easily degraded due to inappropriate thickness of an slurry, inappropriate pressure and improper viscosity of the CNT paste. Moreover, the CNTs in cathode made by the screen-printing method have a random arrangement, rather than a regular array.
In consideration of the adhesion and the arrangement of CNTs, US 2005/0264155A1 discloses a method for producing a carbon nanotube field emission device 100. As shown in FIG. 1, a catalyst film 20 made from Fe, Co, or Ni is deposited on a substrate 10. The substrate 10 having the catalyst film 20 is placed in a reaction furnace at a temperature ranging from 500 to 800° C. to grow a carbon nanotube array 30 thereon. An adhesive 40 is injected into the carbon nanotube array 30. Furthermore, the surfaces of the carbon nanotube array 30 are treated by laser to remove the adhesive 40 so as to ensure exposure of each carbon nanotube. Finally, the carbon nanotube field emission device 100 having improved field emission property is obtained. However, the adhesive 40 coated on the carbon nanotubes can reduce the service life of the device 100.
In addition, Zhu L. et al. (NANO LETTERS, Vol 6, No. 2, pp 243-247, 2006) discloses a transfer method for a carbon nanotube including the following steps: forming a copper foil on a FR-4 board, sputtering an under bump metallization (UBM) layer on the substrate, stencil-printing an eutectic tin-lead solder having a thickness of 100 μm on the UBM layer, transferring a carbon nanotube film on a Si substrate onto the eutectic tin-lead solder, and removing the Si substrate. During transferring, the Si substrate and the FR-4 board were disposed in a reflow oven at 270° C. such that the eutectic tin-lead solder is melted. The melted eutectic tin-lead solder soaked into the carbon nanotube film so as to improve the adhesion between the carbon nanotube film and the FR-4 board. Although the adhesion is improved, the requirement for the reflow oven increases complexity and difficulty of the process. Moreover, only a current density of 5 mA/cm2 was obtained under the test conditions of less than 10−5 Torr and 180 μm spacing between the CNT tip and an anode when an electric field of 3.4 V/μm was applied. The field emission property is insufficient for application and needs further improvement.